Associated with advancing high performance technologies in recent years, electronic element components such as integrated circuits and semiconductor elements have been applied to drive units, semiconductor units, power devices, various electronic equipments and the like used in automobile, robot and other industries. In these units and equipment, an improvement in heat radiating properties has been a major issue. In electronic element components in general, a portion of electric energy is converted into heat energy. Associated with advancement of performance in units and equipments, the amount of the converted heat energy increases, which results in heat generation. Electronic element components are susceptible to heat and may possibly be led to malfunctions and the like by the heat. Thus, it is necessary to eliminate the heat by means of heat radiation, cooling and others.
In electronic equipments and the like with casing, it is possible to effectively eliminate heat by enhancing the thermal conductivity of heat energy generated in the electronic element components to the casing and also scaling up the heat radiating properties of the casing, i.e., a tendency to lose heat by convection and/or radiation. As means to scale up the heat radiating properties of the casing, means applying convection have been primarily used where heat is removed by air sent by use of, for example, a fan. However, associated with downsizing of electronic equipments in recent years, such means are often difficult to be provided in many cases. As a result, a means, for example, has been given attention, in expectation of its radiation effect, where a coating film having excellent heat radiating properties is formed by coating a heat radiating coating material on casings. However, a conventional heat radiating coating material generally includes carbon black or graphite having high radiation rate. Such a heat radiating coating material includes black colored carbon black or graphite, and therefore it is inferior in design and may often cause a problem in designing products of electronic equipments.
Meanwhile, with a view to improving design, heat radiating coating materials colored other than black have been studied. For example, JP H10-279845A (Patent Document 1) describes a far infrared radiation coating material containing metal oxide ceramics such as Al2O3, SiO2 and TiO2. These metal oxide ceramics are in white, thus making the coating material more flexible in color designing as compared with those containing black ingredients and imparting an advantage in improving design. On the other hand, the far infrared radiation coating material described in Patent Document 1 is in white and is opaque. As a result, in the case where an article to be coated is a metal substrate, for example, the coating material is inadequate to be used in expressing the gloss of the metal substrate itself.
JP 2003-309383A (Patent Document 2) describes a heat radiating body having a coating film containing sodium silicate, potassium silicate and metal oxides such as silicon oxide and aluminum oxide. Because the coating film disclosed in Patent Document 2 is a mineral coating material, there is a possibility that its adhesiveness to an article to be coated may be insufficient.
JP 2006-124597A (Patent Document 3) describes a heat radiating material containing a powder of a metal compound selected from a hydrotalcite-series compound, zirconium silicate and zirconium carbide, and a resin ingredient. On the other hand, while Patent Document 3 describes obtaining a transparent coating film, the degree of transparency of the coating film is not disclosed. The degree of transparency of a coating film greatly affects the appearance of a coated article and has a direct effect on design, thus it is an important factor in designing a product.
JP 2004-043612A (Patent Document 4) describes a heat releasing coating material containing SnO2—Sb2O5-based semiconductor particles in a coating material vehicle. The SnO2—Sb2O5-based semiconductor particles used in the patent document are also in white, thus a coating film to be obtained is opaque and imposes a restriction on design.